This invention relates to heat exchangers and in particular to an apparatus for supporting horizontal tubes in a flow of hot gas.
Steam generators frequently include horizontal heating surface forming an economizer, a superheater or a reheater. This surface is normally located in the rear gas pass with hot gas flowing downwardly over the heating surface. Each section normally consists of a plurality of sinuous tubes in parallel flow relationship with one another so that the flow passing through the section is heated to a desired temperature level.
One known method to support these tubes involves the use of vertical hanger tubes passing through the gas pass with the horizontal tubes being attached to some manner to the vertical tubes.
These vertical tubes are normally fluid cooled and must be strong enough to support the accumulated load of all the tubes. Stress concentrations should be avoided on these heavily loaded tubes. The attachment to these tubes is also a source of potential stress concentration due not only to the loading of the tubes but to expansion forces and possible vibration of the supported tube.
Differential expansion of the supported tubes can often be taken by permitting flexing of the support tube where the differential expansion between adjacent supports is small and predictable. The expansion becomes large at certain locations such as between tube banks operating at different temperatures. Also a change from ferritic to austenitic material will create a local sizable expansion difference. Furthermore, two parallel circuits attached between the same pair of hanger tubes may operate at different temperatures and, accordingly, create high stresses in the tubes and in the supports.
It has been known to support these horizontal tubes by welding the tube directly to either the vertical hanger tube itself or a fin on the hanger tube. This requires welding directly on the pressure part which frequently requires a post weld heat treatment. Furthermore, since the beginning and end of a weld tends to be a point of stress concentration, it provides a potential failure area on a pressure part itself.
Alternately, the horizontal tubes have been supported on lugs cantilevered out from the vertical tube without any direct welding between the horizontal and vertical tubes. On occasions, straps have been placed around the tubes to preclude their falling from the support lug. This provides for longitudinal differential expansion of the horizontal tubes. This has produced complex fabrication arrangements and also tends to provide an unfavorable stress concentration on the support lug. The downward loading on the lug leads to a high stress at the upper edge of the lug where it is welded to the support tube. This stress is combined with a weld start or stop point and, therefore, tends to produce a stress concentration at an unfavorable point.